Carbon Nanostructure Embedded Novel Sensor Implementation for Detection of Aromatic Volatile Organic Compounds: An Organized Review

For field-like environmental gas monitoring and noninvasive illness diagnostics, effective sensing materials with exceptional sensing capabilities of sensitive, quick detection of volatile organic compounds (VOCs) are required. Carbon-based nanomaterials (CNMs), like CNTs, graphene, carbon dots (Cdots), and others, have recently drawn a lot of interest for their future application as an elevated-performance sensor for the detection of VOCs. CNMs have a greater potential for developing selective sensors that target VOCs due to their tunable chemical and surface properties. Additionally, the mechanical versatility of CNMs enables the development of novel gas sensors and places them ahead of other sensing materials for wearable applications. An overview of the latest advancements in the study of CNM-based sensors is given in this comprehensive organized review.


INTRODUCTION
Volatile organic compounds (VOCs) are substances that are evaporated and enter the environment under ordinary circumstances. They are present in a variety of products. Due to higher volatility, mobility, and resistance to degradation, VOCs can travel over large distances in the atmosphere. 1 The most common VOCs are halogenated hydrocarbons, e.g., vinyl chloride and chloroethene, as well as aromatic hydrocarbons like: benzene, methylbenzene, xylene, and ethylbenzene.
VOCs come from both natural and man-made sources. Plant emissions, naturally occurring forest fires, and anaerobic moor processes are examples of natural sources. Household and commercial processes that produce VOCs include food processing, the use of fertilizers and pesticides, septic systems, chlorination, transportation, burning hydrocarbon fuels, storing and distributing petroleum, cleaning textiles, printing, the pharmaceutical industry, and more 2 (Figure 1).
Even though VOCs are utilized in many aspects of daily life, their exposure is dangerous. Ocular and sore throats, vomiting, nausea, headaches, loss of coordination, skin allergies, and other symptoms are brought on by small-scale exposure to VOCs. 3 −5 VOCs have been linked to long-term effects in humans, including harm to the central nervous system, reproductive system, lungs, liver, and kidneys. 6 It is generally recognized that some VOCs might cause cancer. 7,8 All living things frequently die when exposed to VOCs over an extended period. The primary causes of global warming are several VOCs, including methane. Additionally, certain VOCs are involved in the destruction of the stratospheric ozone. 9 Significant approaches have been developed to discover VOC analytes in difficult matrices. Gas chromatography−mass spectrometry (GC-MS) is one of the most prominent analytical techniques for identifying VOCs. 10,11 This is because of its consistent accuracy and specificity. Real-time monitoring and analysis of VOC gas have proven difficult due to its high cost, large equipment, and time-consuming sample preconcentration. 12−14 According to their obvious benefits of being portable, affordable, and incredibly sensitive, gas sensors have opened new opportunities for VOC detection in relation to the aforemen-tioned difficulties. This enables the reliability of online analysis and real-time monitoring of VOC analytes. Organic semiconductors, metal oxides, noble metals, sulfides, and carbonbased materials can be used to make functional VOC gas sensors. Carbon materials, particularly those with intrinsic nanoscale characteristics, have been identified as one of the most promising options for detecting VOC gas. 15−18 Nowadays, carbon-based nanomaterials are encouraging the scientific community to make powerful sensor devices due to their characteristic structure, especially graphene and its derivatives such as graphene oxide (GO), CNTs, reduced graphene oxide (rGO), Cdots, and MXene. Several ways of interacting with VOCs, such as π−π stacking, electrostatic forces, and noncovalent bonding, make them suitable candidates for adsorbents. 19 Carbon-based nanomaterials (CNMs) are appealing prospects for the future of automated sensor technology because of their excellent sensing detectability and intriguing transduction capabilities. A majority of atoms in 0−2dimensional CNMs are exposed to the outside environment due to their huge specific surface area and remarkable sensitivity. 20−22 Low-dimensional CNMs like CNTs, graphene, and MXene have a large specific surface area to reach a high detection sensitivity for VOCs; however, the majority of their atoms are discharged into the air. 23, 24 We focused on the most recent developments in CNMs for VOC sensing in this comprehensive review.

GENERAL OVERVIEW OF THE SHAPE AND SIZE OF RGO, CNTS, AND CDS
The carbon nanomaterials can be synthesized with controlled shape and size from 0D to 2D, whereas 3D carbon materials are considered as graphite, diamond, etc. Except for graphite and    25,26 The CNT, which is a circular tube-like structure due to its rolled up graphene sheets, can be a singlewalled or multi-walled CNT. Hence, it is considered a 1D carbon material; its length can be extended to some micrometers long, whereas its diameter can be found to be in the nm range. 27,28 Carbon dots (CDs) are gaining much more attention because of their biocompatibility, excellent physicochemistry, and tunable photoluminescence and are being explored as biosensors. 29,30 They are a new type of carbon nanostructure of >10 nm size and the most water-soluble carbon material. 31 The effective visualization of their shape and size can be studied by a TEM image, which has been shown in Figure 2.
The other derivatives or oxidized forms of graphene by oxygen functional groups such as carboxyl (−C�O), epoxy (−C−O−C−), and hydroxyl (−OH) make graphene oxide suspended in water and other polar media. 35 This degree of oxidation is responsible for the electrical conductivity of modified graphene and the degree of oxidation depending on various synthetic routes. The carbon atoms of modified oxidized graphene are partially sp 3 hybridized and can move above and below the plane. 36 Another derivative of graphene-like sheets can be obtained by reducing graphene oxide through chemical reduction or simple heating methods. Park and Ruoff examined the elemental analysis of carbon and oxygen atoms (atomic C/O ratio, ∼10) for rGO made by combustion techniques and discovered that rGO is not the same as pure graphene since it contains a considerable quantity of oxygen. Furthermore, when cooled to lower temperatures, its conductivity reduces by 3 orders of magnitude, causing it to display nonmetallic behavior. 37 In this way, a very clear difference can be seen between pristine graphene and graphene derivatives such as GO and rGO. Both pristine graphene and its derivatives are twodimensional structures. Still, pristine graphene is relatively inefficient for the adsorption of VOC molecules. In contrast, its derivatives have more adsorption sites due to the presence of different oxygen groups and hence offer high sensitivity toward VOC molecules of the film. 38 On the other hand, the current− voltage (I−V) characteristics for both GO and rGO exhibit ohmic contact, which is suitable for gas-sensing applications. 39

SP 2 -HYBRIDIZED ELECTRONIC BEHAVIOR FOR SENSING
It is very much essential to note the origin of electrical conductivity properties of graphene, CNTs, and Cdots for sensing properties. It will help the understanding of surface interaction as a chemisorption reaction which changes the electrical properties of carbon-based materials. The 2D graphene sheet shows high carrier mobility due to planar sp 2hybridized atomic orbitals, and as a result, reduced graphene oxide (rGO) became a conductive active material for molecular sensors. The rolling form of the graphene sheet (i.e., a carbon nanotube, CNT) results in π confinement due to σ−π rehybridization at its circular curvature structure and creates an asymmetric distribution of σ-bonds and π-orbitals on both sides (inside and outside) of the nanotube, forming a high πelectron conjugation outside the nanotube. 40 Thus, the surface of CNTs can donate or withdraw electrons from any electrondonating or electron-withdrawing entity during molecular interaction, thereby modulating overall electrical resistance. 41,42 Several groups have approached the molecular orbital (MO) theory for the electronic structure of CQDs, which demonstrated that transitions have occurred in n → π* and π → π* as available transition energies. 43−45 According to Larson and Hu, the aromatic sp 2 -hybridized carbons are responsible for their πstates. The presence of π-electron conjugation results in high charge transfer in carbon-based materials. 46 Generally, the detection of VOCs is governed by their interaction with carbonbased materials by diffusion and sensed by the active center of carbon-based materials, 47 Research on developing gas sensors has primarily focused on nanoparticles because of their superior physicochemical characteristics and high surface-to-volume ratio. The recent discovery of CNMs has prompted 73−75 research toward sensing technologies. These materials have been shown to be more effective in producing higher VOC sensors. 76−78 Three steps commonly take place in gas sensors manufactured using CNMs for VOC detection: (i) trapping of VOC gas, (ii) their interaction with the sensing active site, and (iii) dispersing of the VOC gas. Focusing on the complex interactions between the VOC gas and the active center that detects them, CNMs have been used to create a range of VOC gas sensors, including optical 76,79−81 and microgravimetric ones. 77−86 Regarding optical sensors, spectrometric or colorimetric variations in the sensing materials are brought on by the intermolecular interactions among VOC samples and the active center 76,79−81 and resistance-based sensors. 87,78,88−90 and carbon black are examples of zero-, one-, two-, and threedimensional CNMs that have shown such inherent properties that can be easily modified in the development of cutting-edge innovation for sensing applications. Nanomaterials have opened up new pathways and options for sensing analysts or target molecules. 91 Carbon-based nanostructures have several benefits over other commonly used sensor materials, including simple manufacturing procedures, unexpected physiochemical characteristics, greater sensing properties, environmentally friendly materials, improved detection accuracy, high flexibility, sensitivity, and reliability. They also have a large surface-to-volume ratio, high stability, high electrical conductivity, biocompatibility, reduced size, and an improved surface area. CNMs are being studied as a result of further utilization of efficient sensor technology. 92,93 Carbon nanostructures like CNTs and graphene can be used to detect extremely small levels of danger and greenhouse gases. Therefore, using the CNM-based sensor to develop susceptible, small energy gas sensors is both of significant academic interest and of enormous economic significance. 94 The many CNMbased sensors utilized for VOC detection are listed in Table 1 in detail.

CNT-BASED SENSOR FOR VOC DETECTION
A one-dimensional carbon allotrope known as a carbon nanotube (CNT) is composed of sp 2 carbon atoms organized in cylindrical tubes with diameters ranging from 1 to 100 nm. Single-wall carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) are the two types of CNTs used most commonly. MWCNTs, on the other hand, are composed of multiple sheets of concentric single-walled graphene cylinders with an interlayer spacing of 3.4 and are held together by van der Waals forces. 111 SWCNTs are the rolled form of the graphene sheet. Hexagonal rings make up CNTs and control their size, curvature, and electronic properties. Chirality refers to the configuration of carbon hexagonal rings in nanotubes. 112 Since 20 years, CNTs have been made on a massive scale for numerous uses. Arc discharge, 113 CVD, 114 and laser ablation 115 are common techniques for making CNTs. The CVD process and the discharge method are more advantageous for the mass production of high-quality CNTs. 114 One of the oldest clinical practices is breathing analysis used for medical purposes. The binding of volatile compounds to a sensor array, the creation of sensor modifications, which produce distinctive patterns of signals, and the integration of signal patterns allowing categorization are the main phases in the breathing analysis working principle, which also is based on the human olfactory system. 116 A quick, painless, inexpensive, noninvasive method for early disease detection and ongoing physiological monitoring is breathing analysis. 116−118 Analyzing the diaphragmatic movement when breathing can help detect human disorders like atypical sleep problems, asthma, heart attack, and lung cancer early on. 119,120 Due to their appealing qualities, such as good biocompatibility, wearing comfort, low cost, as well as sensitivity to breathing activities in the aspect of lower frequencies and slight amplitude body motions, triboelectric nanogenerators (TENG) have recently been widely used for self-powered respiration monitoring. TENG-based respiration sensors can accurately and continuously track physiological respiratory behaviors and exhaled chemical regents for individualized health care. 121 Further, the 'Liu' research group designed a respiration-driven triboelectric sensor (RTS) for concurrent biomechanical and biochemical breath measurement that can directly transform breath flow into electricity. 122 The Wang research group also created an integrated triboelectric self-powered respiration sensor (TSRS) to track both human breathing patterns and the amount of NH 3 in exhaled gases. 123 'Su' and his research team created a triboelectric self-powered respiration sensor (TSRS) in 2019 to monitor human breathing patterns and the amount of NH 3 in exhaled gases. Figure 3 shows a TSRS that is mounted to a person's chest and tracks their breathing. 124 Su et al. created an alveolus-inspired membrane sensor (AIMS) for self-powered wearable nitrogen dioxide detection and personal physiological evaluation in 2020. This sensor may be used to detect human breath activities for breath analysis. 125 Using a self-validating 64-channel sensor array based on semiconducting single-walled carbon nanotubes, the Panes− Ruiz research group showed precise detection of H 2 S below breathing concentration levels in humid airflow (sc-SWCNTs). The repeatable sensor construction method is based on controlled multiplexing dielectrophoretic sc-SWCNT deposition. The sensing region is developed and produces gold nanoparticles that solve detection at room temperature by leveraging the affinity of gold and sulfur atoms in the gas. The estimated limit of detection (LOD) for sensing devices functionalized with optimum nanoparticle dispersion is 3 ppb, and their sensitivity is 0.122%/ppb. Our sensors surpass certain electrochemical sensors that seem to be available on the market in terms of improved stability and response levels despite selfvalidation. 126 According to the kind and concentration of various surfactants, Chatterjee et al. investigated how several lung cancer VOCs found in human breath impacted the selectivity and sensitivity using CNT-based sensors (nonionic, cationic, and anionic). The performance of the sensors is found to be affected by both the interaction of the surfactant with the analyts and the supramolecular assembly with CNTs. Surfactant−CNT sensors were produced using the spray LbL approach. Water and other polar VOCs, particularly methanol and other alcohols, have demonstrated good sensitivity to CNT-DOC sensors. TX405-CNT sensors were sensitive to three chemicals: benzene, chloroform, and n-pentane. Except for isopropanol, the SDBS-CNT sensors could detect water, acetone, chloroform, and ethanol but not many biomarkers. N-Pentane, acetone, isoprene, and ethanol were all reactive substances for the BnzlkCl-CNT sensors. Although it was shown that pristine CNTs responded best to the majority of the aromatic VOCs in the collection, CTAB-CNT sensors were just moderately sensitive to most VOCs and lacked significant selectivity. 127 Moreover, 'Sinha' research group used zinc oxide (ZnO) and CNTs to create a composite-based chemiresistive sensor. A mechanism has been linked to the VOC adsorption process's temperature-and material-dependent switching. Due to their simplicity in low-temperature synthesis and the large variety of VOC sensing, stability, or other advantageous qualities, ZnO/ CNT composites have been selected in this work as a crucial material above other metal oxides. Additionally, it is anticipated that a p−n heterojunction will develop at every point in which CNT and ZnO are in contact. This will also impact the composite sensor's ability to sense. A potential is created as an outcome of the interaction of n-type (ZnO) and p-type (CNT) materials. As a result, if VOCs were adsorbed onto the surface of a composite sensor, their potential should decrease. 128 The total sensor system also benefits from this drop in junction potential. Figure 4 depicts an electron transport route schematically in connection to the phenomenon. 129 For the purpose of detecting volatile VOCs, the Turner research group developed vapor quantum resistive sensors (vQRSs) utilizing random networks made of carbon nanotubes (CNTs) that have been functionalized with caffeine. Energydispersive X-ray spectroscopy (EDS) and a localized transmission electron microscopy (TEM) image both demonstrate that caffeine has moved to the CNT−CNT junction (CNTj). Furthermore, at the CNT intersection, we conducted localized EDS (Figure 5a,b). According to the elemental analysis, about 5% of atomic nitrogen is present. Such findings prove that  caffeine molecules can cluster at the CNTj of random networks and thus are noncovalently bound to CNT surfaces.
Specific VOCs like acetone, toluene, and ethanol have increased sensitivity and distinct selectivity and a quick response time with caffeine-enhanced CNT junction (Caf-CNTj) vQRSs. According to molecular dynamics simulations, the sensor's sensitivity is diffusion-limited, with ethanol actively contributing (MD). The created Caf-CNTj sensors could be a strong contender for various activities performed by an electronic nose, including breath analysis and food quality monitoring. 130 Bohli et al. created a room-temperature toluene and benzene sensor in 2019 using MWCNTs functionalized with a long-chain thiol self-assembled monolayer, 1-hexadecanethiol (HDT), and decorated with Au nanoparticles. The thiol monolayer adhering to the MWCNTs was examined using FT-IR and highresolution TEM, which were used to describe the gold nanoparticle decorating. The ability of Au-MWCNT and HDT/Au-MWCNT sensors to detect VOCs at levels as low as ppm is evidence that the self-assembled layer enhances the sensing selectivity, sensitivity by a factor of 17, and response dynamics. The response of the Au-MWCNT sensor to various vapor injection amounts for toluene and benzene at room temperature is shown in Figure 6a,b. 131 NiWO 4 microflowers (MFs) were used to embellish MWCNTs in composites for the detection of NH 3 . The material's porous nature, the large specific surface area, and the p−n heterojunction formed by the MWNTs and NiWO 4 contribute to the better sensing capabilities of this composite.
The sensor based on 10% (MWN10) has the best gas sensitivity, with a sensitivity of 13.07 to 50 ppm of NH 3 at room temperature and a detecting lower limit of 20 ppm, according to the gas sensitivity of the sensor based on daisy-like NiWO 4 / MWCNTs. 132

GRAPHENE-EMBEDDED SENSOR FOR VOC DETECTION
Graphene has garnered a great deal of scientific attention since Novoselov and associates used mechanical exfoliation to synthesize it for the first time in 2004, 133 winning them the Nobel Prize for their contribution to Physics in 2010. 134 Pure graphene and graphene oxide (GO) are the family members of "graphene". Graphene is a single layer of graphite formed of predictable hexagonal arrangements of 2D carbon atoms. This arrangement is identical to that of graphite. Graphene is relatively light, weighing just around 0.77 mg per square meter. 135 Smart electronic material 'Graphene' has many unique properties and distinguishing characteristics. The structure's extremely high electron mobility (200 000 cm 2 V 1 s 1 ), much more significant than carbon nanotubes (CNTs), is made possible by its zero band gap. 136 It possesses exceptional mechanical strength, an extremely high surface-to-volume ratio, 137,138 high capacitance, 139,140 outstanding thermal conductivity, 141 exceptional electrical conductivity, and the possibility for atomically clean graphene sheets on the graphene lattice. 142 In addition to such amazing properties, graphene also enables sensitive analyte detection because of its exceptionally low electrical noise. 143 Functionalized reduced graphene oxide (RGO) VOC sensors for detecting the ppm level of VOCs was developed by Tombel et al. (Figure 7). 144 Using a Ti/Pt integrated electrode and functionalized nanomaterial on RGO thin film, a single semiconductor sensor was assembled on a SiO 2 /Si substrate (IDE). The three VOCs that were the focus of the detection studies were acetone, toluene, and isoprene. These experiments were conducted at ambient temperatures of 30°C and 40% relative humidity. 145 Reduced graphene oxide (rGO)/metalloporphyrin-based VOC sensors are simple and affordable to make and can precisely detect VOCs linked to a number of diseases often present in human breath. They have been described by the Lee  research group. Using a drop casting technique, we created an rGO-metalloporphyrin-sensing array and examined it with EDS and scanning electron microscopy (SEM). Next, three diseaserelated breath VOCs (acetone, ammonia, and isopropanol), as well as carbon monoxide, were introduced to the detecting array. 146 ZnO nanowire−rGO nanocomposites are created by combining ZnO nanowires and graphene oxide (GO) in various ratios. This nanocomposite is utilized as a sensor to detect NH 3 , and it performs substantially superior to pure reduced grapheneoxide-based gas sensors, exhibiting an outstanding response (19.2%) to NH 3 at ambient temperature. 147 Gupta et al. investigated the use of thin coating rGO as a sensing material on a QCM sensor to detect VOC vapors at ambient temperature. To make the rGO suspension, the aqueous suspension of GO paste is first reduced with ascorbic acid. The rGO thin films were cast using the drop-cast technique and dried at room temperature. For their contaminant-free wrinkled form, multilayered graphene structure, high mobility of 2 × 10 4 cm 2 /(V s), and low resistivity of 4 × 10 −1 cm, acetone molecules are more readily absorbed on the surface of rGO thin films. As a result of the rGO sensing material used in the QCM gas sensor, rapid reactions are possible. The QCM gas sensors' quick reaction and recovery periods of 20−30 s have been proven using commercially available rGO thin sensing films. 148 It is claimed that to produce nanostructured detectors GO nanodomains of p-type were carefully inserted into an n-type 3D ZnO nanoarchitecture. These ultraporous nanoheterojunction networks' features were investigated using physical and chemical approaches, demonstrating how GO affects the networks' ability to sense chemicals and light. By employing UV light activation of the sensing reactions, these nanocomposite materials were also employed to detect many common VOCs, including ethyl alcohol, propanone, and ethylbenzene, down to ambient temperature. Here, Figure 8a−d shows how exposure to UV light, temperature, and pure ZnO or 32:1 ZnO/GO films' chemical sensing affects reactions to acetone. 149 The two main forms of sensitizing agents, N-doped graphene quantum dots (N-GQDs-NSs) coupled to nanosheets and Ndoped carbon nanoparticles (N-CNPs) (Figure 9), have been produced from natural carbon powder utilizing the straightforward methods of oxidation and centrifuge separation. Analysis using the techniques of XRD, UV−vis, FT-IR, FE-SEM, XPS, Raman spectroscopy, AFM, HR-TEM, and FL revealed that nitrogen had been successfully incorporated into carbon nanoparticles, providing them an average plane dimension of  50 nm and a reasonably smooth surface. The capacity of the produced samples to identify volatile organic substances using a simple optical-fiber-based sensor setup was used to create and establish the samples' adaptability for sensitizing agents. Comparative laboratory studies have explored that the recommended sensor's efficiency can respond quickly within just a few tens of seconds when exposed to methanol vapor. When exposed to various alcohol vapors in an atmosphere with ambient air, their lower limits of detection were, respectively, 4.3, 4.9, and 10.5 ppm. 150 For rapid identification of VOCs, field effect transistors (FETs) with a p-TiO 2 nanoparticle (NPs)/GO heterojunction are used. P-type anatase TiO 2 NPs made from sol−gel were used as the channel materials and were implanted in a few stacked GO channels. To determine the ideal ratio of GO and TiO 2 NPs within the hybrid channel, extensive microscopic, spectroscopic, and electrical characterizations were carried out. On a SiO 2 /Si substrate, a back-gated FET sensor was created and put through testing while being exposed to various VOCs. At 100°C, tests with p-TiO 2 /GO FET sensors at zero gate voltage (VGS = 0) demonstrated ethanol selectivity. The lower detection limit of ethanol was enhanced to 500 ppb by utilizing an appropriate gate voltage (VGS > 0, near the Dirac point), significantly enhancing its sensitivity and p-TiO 2 −GO hybrid for fieldassisted sensitivity amplification. Transient behavior in the presence of reducing vapor ethanol, on the other hand, served as evidence for the p-type conductance of the composites of p-TiO2 NPs and p-GO. The pure GO sensor (S9) could not respond at 100 ppm, but the S1−S8 sensors produced a response within the dynamic range of 25−300 ppm. Every sensor demonstrated a stable baseline with predictable sensing behavior ( Figure 10). 151 A susceptible HCHO sensor is suggested and demonstrated using a heterostructure of the Sn 3 O 4 /rGO composite. It features a low working temperature and a wide detection range. The hydrothermal Sn 3 O 4 /rGO composite exhibits a large specific surface area and a microstructure like a flower. The sensing features of sensors based on pure Sn 3 O 4 and Sn 3 O 4 /rGO composites are thoroughly analyzed to learn how the heterostructure influences the sensing performance of HCHO. According to the observed data, the Sn 3 O 4 /rGO composite sensor has a significantly higher sensing response at a lower working temperature than the pure Sn 3 O 4 sensor. Figure 11a,b presents data analysis and the schematic configuration of the gas sensor. 152 As one of the most frequently harmful irritating gases, NO 2 can cause asthma and other illnesses in people. 153 Researchers are concentrating on creating light, affordable wearable electronics with excellent performance, and adaptability due to the recent rapid advancement of gas sensor research. 154 Chen and a colleague created a smartphone-enabled, completely integrated wireless system for real-time NO 2 monitoring using a flexible ZnS nanoparticle/nitrogen-doped reduced graphene oxide (ZnS NP/N-rGO) sensor. The device exhibited a response of 2.2−10 ppm of NO 2 , a rapid recovery performance of 724 s, a power consumption of 0.52 W, and an extremely low theoretical limit of detection (69 ppb) for graphene-based sensors. 155

CD-BASED SENSOR FOR VOC DETECTION
For their excellent environmentally friendly nature, brightness, tunable fluorescence, inertness, low cost, straightforward synthetic route, and availability for a wide range of starting  materials, carbon dots (Cdots), an emerging nanomaterial of the carbon-based materials family, have attracted significant research interest. 156 For use in acetone gas-sensing applications, the Mishra research group reports producing ZnS quantum dots (QDs) via a hot-injection technique (Figure 12). The generated ZnS QDs were characterized using XRD and TEM analysis. The ZnS QD sensor exhibits rapid response and recovery times. At a 100 ppm acetone concentration at 175°C, it also demonstrates outstanding stability, high sensitivity, and strong selectivity. Table 2 also compares the ZnS nanomaterial-based acetone    157 An acetone sensor was created using a quartz crystal microbalance (QCM), which was modified with graphene quantum dots (GQDs) to sense gases. GQDs were produced via citrate pyrolysis, and their characteristics were determined by high-resolution TEM (HR-TEM). They looked at the sensor's gas sensitivity to acetone at low concentrations. With a sensitivity of 16.78 Hz/ppm and a minimal detection limit of 2.5 ppm, it demonstrated good linearity at less than 240 ppm acetone concentrations. The sensor showed high acetone selectivity in a combination of acetone, butanol, and isopropanol ( Figure 13). The same sensor's response time was constant for varying acetone concentrations, and the response and recovery durations were 32 and 48 s, respectively. 166

OTHER CARBON NANOMATERIAL-BASED SENSORS FOR THE DETECTION OF VOCS
The gas-sensing properties of MXene may be predicted using the quantum mechanical approach and basic principles based on density functional theory (DFT). Using the Schrodinger equation, DFT-based first-principles examine the electronic structure of materials. 167 The absorption energy of material to certain gas molecules, as well as the associated charge transfer characteristics, are important factors in predicting sensing qualities based on gas sensing. Yu 169−171 By chemically treating precursors of the M n+1 AX n phase, also known as the MAX phase, where M is an early transition metal, A is typically an element IIIA or IVA; X is C and/or N; and n = 1, 2, and 3 MXenes are produced. 172 MXenes are a focus of research because of a number of intriguing characteristics that promise to lead the way for future sensor development. A fascinating class of two-dimensional materials called MXenes has lately attracted interest for their possible use in gas sensing. 173 Here, we show the efficiency of a new Mo 2 CT x MXene sensor in detecting VOCs. On a Si/SiO 2 substrate, the suggested sensor is a chemiresistive device made by photolithography. The effectiveness of the sensor is examined in relation to different MXene manufacturing conditions. Different VOC concentrations, including ethanol, toluene, benzene, acetone, and methanol, are examined at room temperature. The effectiveness of several MXene-based gas sensors is compared in Table 3. 174 In this case, the model materials for hybridization and their application to the detection of different volatile organic chemicals are Ti 3 C 2 T x and WSe 2 ( Figure 14). The Ti 3 C 2 T x / WSe 2 hybrid sensor has excellent adaptability for a wide range of volatile organic compounds, low noise levels, and exceptionally fast response and recovery times. The hybrid sensor's sensitivities to ethanol have risen about 12-fold when compared to pure Ti 3 C 2 T x . Furthermore, by restricting the contact of water molecules from Ti 3 C 2 T x 's edges, the hybridization process offers a successful defense against MXene oxidation. Detecting volatile organic molecules containing oxygen is highly sensitive and selective, and an enhanced method for Ti 3 C 2 T x /WSe 2 heterostructured materials is presented. 173 It is highlighted how well the Zhao research team's novel MXene, V 4 C 3 T x , functions as an acetone sensor. It was made from V 4 AlC 3 by selectively etching the Al layer with aqueous HF RT. A V 4 C 3 T x -based acetone sensor operates well due to its low operating temperature of 25°C, low detection limit of 1 ppm below the 1.8 ppm diabetes diagnostic threshold, and good selectivity toward acetone in a mixed mixture of water vapor and acetone. It might lead to a much faster and earlier diagnosis of diabetes. V 4 C 3 T x MXene is used for the first time in this work in the context of acetone detection. 180 We provide a simple solvothermal method for fabricating W 18 181 Schottky-barrier-equipped Ti 3 C 2 T x −ZnO nanosheet hybrids have been produced for NO 2 recovery and detection under UV light. By using HF solution to etch the Ti 3 AlC 2 Al layer, Ti 3 C 2 T x nanosheets were produced (MAX). ZnO nanosheets' porous structure is crucial for the adsorption of NO 2 gas. With the aid of UV irradiation during the recovery process, the Ti 3 C 2 T x −ZnO nanosheet-based gas sensor displayed better NO 2 -detecting skills, including a high sensitivity of 367.63% to 20 ppm of NO 2 and a quicker response/recovery time of 22 s/10 s. 182 The first oxygenated amorphous carbon (a-CO x )/graphite (G) nanofilament-based buckypaper sensor was developed by Homaeigohar ( Figure 15). By forming hydrogen bonds, oxygencontaining groups influence the group's capacity to extract electrons, the density of hole carriers, and, therefore, the resistivity, which in turn affect the group's selectivity toward VOCs like ethanol and acetone. However, the creation of charge-transfer complexes caused by the toluene aromatic ring's electrostatic interactions with the electrons of the graphitic crystals may be the main cause of the sensor's great responsiveness to nonpolar toluene. 183

CONCLUSIONS, OUTLOOK, AND PERSPECTIVE
The development of VOC sensing utilizing CNMs is outlined in this comprehensive review. Due to the wide range of uses for VOC environmental monitoring, the difficulty of selecting VOC detection is growing every day. Toxic VOC detection is crucial for protecting the environment and human health. Due to their unique morphology and features, developing CNMs has been the foundation for sensor technology over the preceding two decades. CNMs like graphene, CNTs, and their derivatives, which have a large number of adsorption sites, low density, tunable electrical properties, high carrier mobility, low operating temperatures, longer lives, and ease of recovery, are appropriate sensing materials for a variety of toxic pollutant gases and volatile organic compounds. Numerous researchers have observed an increase in sensitivity that allows for detecting harmful VOCs at ppb levels. The selectivity of a CNM-based sensor toward a specific VOC at room temperature has been shown in numerous papers. According to the literature, metal oxide and nanocarbon  hybrids have outstanding sensing capabilities, with nanocarbon serving as the primary component. Composites made of nanocarbon have shown a strong potential for use in VOC sensing.
However, some measure of research should be focused on the synthesis or design of porous or hierarchical structures of carbon-based materials for the suitable VOC molecule adsorption with more reaction sites which can help with a better understanding of structural-adsorption/absorption property relationships that will garnish sensing mechanisms. Also, by analysis of theoretical calculation, the change of electronic properties of host−guest items can be explored for selective gas sensors.